Tilted Grating Sensor

ABSTRACT

The present invention relates to a sensor using a tilted fiber grating to detect physical manifestations occurring in a medium. Such physical manifestations induce measurable changes in the optical property of the tilted fiber grating. The sensor comprises a sensing surface which is to be exposed to the medium, an optical pathway and a tilted grating in the optical pathway. The grating is responsive to electromagnetic radiation propagating in the optical pathway to generate a response conveying information on the physical manifestation.

FIELD OF THE INVENTION

The invention relates to a sensor using a tilted fiber grating to detect external events that induce measurable changes in an optical property of the tilted fiber grating. The sensor can be used to sense a wide range of biological processes, biological elements, chemical elements or manifestations of physical phenomena such as temperature, strain or index of refraction.

BACKGROUND OF THE INVENTION

Fiber Bragg grating (FBG) sensors have a wide range of applications such as pressure-strain sensors, temperature sensors, micro-bending sensors and external refractive index sensors. As these optical sensors are inherently immune from electromagnetic interference and chemically inert, they are very attractive in bio-chemical applications and hazardous surroundings.

The sensing mechanism most often used in FBGs arises from the fact that the reflection wavelength for the forward propagating core mode varies linearly with temperature and strain. Since the wavelength can be measured with an accuracy of 10 pm relatively easily near 1550 nm, this represents a relative resolution of about 6 ppm. A variant of the same concept uses so-called Long Period Gratings (LPG) where coupling occurs between the forward propagating core mode and forward propagating cladding modes. In this case, the sensitivity of the resonance wavelength to perturbations can be greatly enhanced for some of the cladding modes. Furthermore, since LPGs involve cladding modes there has been great interest in using these for refractive index sensing by immersing the fibers in the medium to be measured. Special absorbing coatings can also be used to detect chemicals or liquids through the refractive index changes (or volume changes) induced in the coatings. However, the spectral response of LPG resonances is rather broad (width greater than 10 nm) making high accuracy measurements of small wavelength changes more difficult than with FBGs. Ideally, such refractive index sensors should be able to distinguish different kinds of perturbations, and insensitivity to temperature is often particularly desirable. A problem with both FBG and LPG sensors is that they are intrinsically quite sensitive to temperature, with resonance wavelengths drifting by about 10 pmfC, unless special bulky packaging is used to athermalize the device. In order to circumvent this problem in refractive index sensors, devices proposed so far have involved combination of gratings in one sensor such as two different types of fiber Bragg gratings, two fiber Bragg gratings with different cladding diameters and a long period grating (LPG) with a Bragg gratings. In such cases, the differential sensitivity of the two gratings to temperature and the desired measurand is used to discriminate between the two perturbations.

Against this background it can be clearly seen that the current sensor technology has drawbacks. It is therefore the aim of the present invention to alleviate those drawbacks.

SUMMARY OF THE INVENTION

As embodied and broadly described herein the invention provides a sensor for sensing at least one physical manifestation occurring in a medium. The sensor comprises a sensing surface for exposure to the medium, an optical pathway and a tilted grating in the optical pathway. The grating is responsive to electromagnetic radiation propagating in the optical pathway to generate a response conveying information on the at least one physical manifestation.

As embodied and broadly described herein the invention also provides a sensor for sensing a physical manifestation occurring externally of the sensor. The sensor comprises a sensing surface for exposure to the physical manifestation, an optical pathway and a tilted grating in the optical pathway. The tilted grating is responsive to electromagnetic radiation propagating in the optical pathway to induce SPR adjacent the sensing surface.

As embodied and broadly described herein the invention also provides a method for detecting the presence of bacteria in a medium. The method comprises providing a sensor having a sensing surface, the sensor also having an optical pathway containing a tilted grating, placing the sensing surface in contact with the medium and determining from a response of the sensor if the bacteria is present in the medium.

As embodied and broadly described herein the invention also provides a method for detecting the presence of virus in a medium. The method comprises providing a sensor having a sensing surface, the sensor having an optical pathway containing a tilted grating, placing the sensing surface in contact with the medium and determining from a response of the sensor if the virus is present in the medium.

As embodied and broadly described herein the invention also provides a method for measuring the concentration of sugar in a medium. The method comprises providing a sensor having a sensing surface, the sensor having an optical pathway containing a tilted grating, placing the sensing surface in contact with the medium and determining from a response of the sensor the concentration of sugar in the medium.

As embodied and broadly described herein the invention also provides a method for measuring the concentration of alcohol in a medium. The method comprises providing a sensor having a sensing surface, the sensor having an optical pathway containing a tilted grating, placing the sensing surface in contact with the medium and determining from a response of the sensor the concentration of alcohol in the medium.

As embodied and broadly described herein the invention also provides a method for detecting the presence of a chemical or biological element in a medium. The method comprises providing a sensor having a sensing surface, the sensor having an optical pathway containing a tilted grating, placing the sensing surface in contact with the medium and determining from a response of the sensor if the chemical or biological element is present in the medium.

As embodied and broadly described herein the invention also provides a method for measuring a degree of curing of a curable material. The method comprises the steps of providing a sensor having a sensing surface, the sensor having an optical pathway containing a tilted grating, placing the sensing surface in contact with the curable material determining from a response of the sensor the degree of curing of the curable material.

As embodied and broadly described herein the invention also provides an elongation strain sensor. The elongation strain sensor comprises an optical pathway and a tilted grating in the optical pathway to generate a response conveying information on elongation strain acting on the sensor.

As embodied and broadly described herein the invention also provides a method for measuring elongation strain. The method comprises receiving a response from a sensor containing a tilted grating subjected to elongation strain, the response conveying information on reaction of the tilted grating to elongation strain and on reaction of the tilted grating to temperature, processing the response of the tilted grating to distinguish the reaction of the tilted grating to elongation strain from the reaction of the tilted grating to temperature.

As embodied and broadly described herein the invention also provides an apparatus for measuring elongation strain. The apparatus comprises an elongation strain sensor having an optical pathway, a tilted grating in the optical pathway to generate a response conveying information on elongation strain and temperature acting on said sensor and a signal processing unit to process the response of the tilted grating and distinguish in the response to reaction of the tilted grating to elongation strain from the reaction of the tilted grating to temperature.

As embodied and broadly described herein the invention also provides a bending strain sensor. The bending strain sensor comprises an optical pathway and a tilted grating in the optical pathway to generate a response conveying information on bending strain acting on the sensor.

As embodied and broadly described herein the invention also provides a method for measuring bending strain. The method comprises receiving a response from a sensor containing a tilted grating subjected to bending strain, the response conveying information on: reaction of the tilted grating to bending strain; reaction of the tilted grating to temperature. The method also comprises processing the response of the tilted grating to distinguish the reaction of the tilted grating to bending strain from the reaction of the tilted grating to temperature.

As embodied and broadly described herein the invention also provides an apparatus for measuring bending strain. The apparatus comprises a bending strain sensor, having an optical pathway, a tilted grating in said optical pathway to generate a response conveying information on bending strain and temperature acting on said tilted grating, a signal processing unit to process the response of the tilted grating and distinguish in the response to reaction of said tilted grating to bending strain from the reaction of the tilted grating to temperature.

As embodied and broadly described herein the invention also provides a pressure sensor. The pressure sensor comprises an optical pathway, a tilted grating in the optical pathway to generate a response conveying information on bending strain acting on said sensor, a flexible member, the optical pathway being mounted to the flexible member. The flexible member flexes in response to pressurized fluid and induces in the tilted grating bending strain.

As embodied and broadly described herein the invention also provides a sensor, for sensing at least one physical manifestation occurring in a medium. The sensor comprises an optical pathway an interface coupled with the optical pathway, the interface being responsive to the physical manifestation to induce strain on said optical pathway; a tilted grating in the optical pathway, the tilted grating being responsive electromagnetic radiation propagating in the optical pathway to generate a response conveying information on the strain induced on the optical pathway.

BRIEF DESCRIPTION OF THE DRAWINGS

A detailed description of examples of implementation of the present invention is provided hereinbelow with reference to the following drawings, in which:

FIG. 1 a is a high level schematic of a TFBG sensor;

FIG. 1 b is a graph showing the simulated transmission spectrum of a TFBG;

FIG. 2 a is a graph showing the differential wavelength shift for selected TFBG resonances as a function of the refractive index of the surrounding medium for a 125 μm cladding diameter;

FIG. 2 b is a graph showing the differential wavelength shift for selected TFBG resonances as a function of the refractive index of the surrounding medium for an 80 μm cladding diameter;

FIG. 3 is a graph showing the experimental TFBG transmission spectrum for three values of SRI;

FIG. 4 is a graph showing the temperature sensitivity of several TFBG resonances and differential shift from the core mode resonance;

FIG. 5 is a graph showing the effect of bending of a TFBG on its transmission spectrum;

FIG. 6 is a scheme for interrogating an individual cladding mode resonance using a photo-detector collecting all the reflected light. The insets in the figure indicate the transmission spectrum of a typical TFBG (near a cladding mode resonance) and the reflection spectrum of the interrogating Fiber Bragg Grating (FBG). The position of the FBG reflection peak determines which cladding mode is interrogated.

FIG. 7 is a graph showing the displacement of the reflected power from a TFBG-FBG pair when the FBG is tension-tuned across a TFBG cladding mode resonance.

FIG. 8 is a graph showing the transmission spectrum of a TFBG;

FIG. 9 is a cross-sectional view of optical fiber containing a TFBG that produces surface plasmon resonances;

FIG. 10 shows a dielectric/conductor interface on an optical fiber to illustrate the Kretchman configuration for exciting SPR.

FIG. 11 is a graph illustrating the transmission spectrum of the same grating as the one of FIG. 8, but with a 20 mm gold layer in air.

FIG. 12 is a graph showing the transmission spectrum of gold-plated TFBG in a sugar solution. The bracket identifies the peak position of the anomalous resonance.

FIG. 13 is a graph showing the SPR peak wavelength (λ_(p)) and corresponding cladding mode effective index on the refractive index of the external medium at 589 nm (n_(D))

FIG. 14 is a block diagram of a measuring apparatus using a tilted grating, according to an example of implementation of the invention.

FIG. 15 is a block diagram of the signal processing device of the measurement apparatus shown in FIG. 14.

FIG. 16 is a scheme of a diaphragm 2000 that is made of flexible material and exposed to pressure on its side 2002. A sensor 2004 is mounted to the diaphragm. The sensor is placed on the convex side of the diaphragm 2000.

FIG. 17 is a schematic representation of self-assembled monolayers molecules on the sensing surface. The self-assembled monolayers molecules comprises a recognition analyte which associates with an analyte of interest.

FIG. 18 is a block diagram of a measuring apparatus using a tilted grating, according to an example of implementation of the invention. The apparatus comprises a reflector as well as an optical coupler.

In the drawings, embodiments of the invention are illustrated by way of example. It is to be expressly understood that the description and drawings are only for purposes of illustration and as an aid to understanding, and are not intended to be a definition of the limits of the invention.

DETAILED DESCRIPTION

A non-limiting example of implementation of this invention uses an optical structure in which light is guided by a core medium able to support one or few guided modes, surrounded by a finite-sized cladding medium whereas the cladding itself acts as a multimode waveguide. Specific examples of this structure include optical fibers and planar light circuit (PLC). In a weakly tilted fiber Bragg grating (TFBG) sensor both a core mode resonance and several cladding mode resonances appear simultaneously, as shown in FIG. 1. Weakly tilted gratings are defined as having a tilt angle greater than zero but less than 45 degrees relative to the propagation axis, so that there is a non-zero core mode reflection induced by the grating. In a specific example, the tilt angle is in the range from greater than zero and about 20 degrees. In a more specific example the tilt angle is in the range from about 2 degrees to 12 degrees. This has several advantages. The cladding mode resonances are sensitive to the external environment (refractive index, deposited layer thicknesses, etc.) and to physical changes in the whole fiber cross-section (shear strains arising from bending for instance), while the core mode (Bragg) resonance is only sensitive to axial strain and temperature. The temperature dependence of cladding modes is similar to that of the core modes, so that the effect of temperature can be removed from the cladding mode resonance by monitoring the wavelength difference between the core mode resonance and selected cladding mode resonances. Using this technique, sensors can be made for sensing physical manifestations such as elongation strain, bending, or measuring the Surrounding Refractive Index (SRI), that are temperature-independent.

Another feature which may constitute an advantage of the TFBG over the LPG to couple to cladding modes is that the resonances are as narrow as those of a FBG, i.e. of the order of a few hundred pm, instead of several tens of nm for LPGs. Therefore, wavelength interrogation for TFBG occurs over narrow wavelength ranges and makes it possible to use commonly available light sources, detectors, couplers and multiplexers. In particular, a typical entire TFBG spectrum fits easily within the standard telecommunication bands (1530-1560 nm). Also, by using the resonance positions and/or strengths individually, allows extracting multiple sensing parameters from a single sensor. More specifically, a multi-functional sensor can be built by monitoring several cladding mode resonances (or groups of resonances) which have different sensitivities to different physical manifestations. For instance, bending affects mainly low order cladding modes, while SRI variations affect higher order cladding modes preferentially, and neither perturbation affects the core mode resonance.

As is well known, the Bragg reflection and cladding mode resonance wavelengths λB and λiclad of TFBG are determined by a phase-matching condition and can be expressed as follows:

λ_(B)=2n _(eff)Λ/cos θ  (1)

λ^(i) _(clad)=(n ^(i) _(eff) +n ^(i) _(clad))Λ/cos θ  (2)

where neff, nieff and niclad are the effective indices of the core mode at λB and the core mode and the ith cladding mode at λiclad respectively, and Λ and θ are the period and the internal tilt angle of the TFBG. The tilt angle is defined as the angle formed by the TFBG and the imaginary axis of the optical pathway containing the TFBG along which the optical signal interrogating the TFBG propagates. When the optical pathway is defined by an optical fiber, the axis of the optical fiber will usually constitute the axis of optical signal propagation. For TFBG structures, if only the Bragg and cladding mode wavelength shifts (ΔλB, Δλiclad) caused by external refractive index changes (Δnext) and temperature changes (ΔT) are taken into account, the wavelength shifts ΔλB and Δλiclad can be written from equations (1) and (2) as follows:

In standard optical fibers, the effective index of core modes (n_(eff), n^(i) _(eff)) are insensitive to the external refractive index changes, so the equations (3) and (4) may be simplified as following:

Equations (5) and (6) show that the cladding resonances will change with the external refractive index, but not the Bragg wavelength. It can also be seen that the different cladding modes may have different sensitivities to SRI changes. The differential wavelength shift, which can be used as a sensing quantity, is given by Equation (7). The second term on the right-hand-side of Equation (7) represents the temperature dependence of the relative wavelength shift. Using the thermal expansion coefficient of silica (0.55×10⁻⁶/° C.) and an extreme worst case estimate of 1×10⁻⁶/° C. for the difference in the temperature dependence of the refractive indices of the core and cladding glasses (which change individually by about 10×10⁻⁶/° C.)), it can be shown that the temperature dependence of the differential resonance is less than 0.54 pm/° C. for the differential wavelength shift.

Accordingly, the value of Δn_(ext) determined from Equation (7) is the value for the SRI at the sensing temperature (and the temperature can be determined independently by the core mode resonance value from Equation (5)).

A number of simulations of TFBG constructions will now be discussed to illustrate the main features of the TFBG sensor. Those simulations have been made using a commercial optical fiber grating software simulator, such as OptiGrating version 4.2 from Optiwave Corp., (http://www.optiwave.com).

First, a TFBG with grating period of 534 nm and internal tilt angle of 4.5° was simulated in a CORNING SMF-28 standard optical fiber as a function of the refractive index of the external medium (SRI). The structure of such TFBG grating is shown at FIG. 1 a. Broadly speaking, the TFBG is integrated into an optical pathway 10 which is in the form of an optical fiber. The optical pathway has a core 12 surrounded by a cladding 14. The TFBG grating 16 is written into the core 12. The axis of the TFBG 16, which is perpendicular to the bars forming the grating 16 is at an angle of 4.5° with relation of the axis 18 of the optical pathway 12, along which an optical signal interrogating the TFBG 16 propagates. When the TFBG 16 is interrogated by an optical signal the TFBG 16 produces a response that has two components. One of the components is the core mode resonance 20 which is reflected back toward the source of the optical signal. The other is the cladding mode resonance 22 which includes a series of individual emissions at different wavelengths that propagate in the cladding 14 toward the outer surface of the optical pathway.

FIG. 2( a) shows the wavelength shift of 5 different cladding mode resonances (relative to the core mode resonance, which remains fixed) when the SRI changes from 1.33 to 1.44. The highest order cladding modes begin to shift sooner as the SRI increases because their mode field intensity extends further into the external medium as they are closer to their cut-off. High order modes sequentially disappear (from the short wavelength side, as shown in FIG. 1 b) when the SRI reaches values for which they are cut-off.

Note that the final slope before cut-off for the various resonances appears to converge for all the modes to a common value near 180 pm/% SRI. Therefore, monitoring several resonances simultaneously allows high sensitivity measurements of SRI over a larger range of values.

Higher sensitivity also can be achieved by reducing the cladding layer thickness (by etching in diluted hydrofluoric acid for instance), or by using fibers that are fabricated with smaller diameter claddings: this results in fewer cladding modes that are more widely separated in wavelength. If the cladding layer diameter is reduced from 125 μm to 80 μm, the SRI sensitivity becomes 350 pm/% SRI, as shown in FIG. 2( b). By further reducing the cladding layer diameter, much higher wavelength shifts can be expected. Note that a potential problem may arise if the cladding layer diameter is reduced to less than 30 μm; at or near this value the Bragg wavelength becomes sensitive to the SRI changes and the in-fiber temperature reference may be lost. Another option is to use functional thin films to “pull” certain modes out of the fiber cladding and make them sensitive to specific environments.

The results of experimental work conducted for SRI sensing and temperature insensitivity are shown in FIGS. 3 and 4 respectively. The TFBG is written in a CORNING SMF 28 fiber using ArF excimer laser light at 193 nm and a phase mask to generate the grating pattern. This is not to be considered a restriction since any method of fabrication for FBGs is applicable to the sensors described here. The tilt angle was adjusted experimentally until the core and main cladding mode resonance transmission dips achieved comparable attenuation levels. The exact value of the tilt angle is not a critical parameter as it only influences the relative strengths of the core and cladding mode resonances for a given sensor (and hence only impacts the details of the sensor interrogation scheme chosen in a particular case). The SRI sensitivity was obtained by measuring the transmission spectra of the TFBG immersed in calibrated refractive index fluids (from Cargille Laboratories). FIG. 3 shows the changes in transmission of the highest order cladding modes with SRI of 1.40, 1.42 and 1.44, illustrating that resonance shifts increase with mode order and that large changes occur when modes approach cut-off.

The measured wavelength shifts of modes approaching cut-off are 0.275 nm and 0.218 nm when the SRI changes from 1.40 to 1.42 and 1.42 to 1.44 respectively. The latter shift corresponds to a sensitivity of 300 pm/% SRI, actually higher than the theoretical values indicated previously. If the power level within a narrow band at a fixed wavelength is monitored instead of the wavelength itself, the above mentioned wavelength sensitivity translates into a change in transmitted power of 20% for a change in SRI of 0.7% since resonances are at most 200 pm wide for 1 cm-long gratings, and several resonances have an amplitude of about 1 dB. By monitoring simultaneously the power level near the core mode resonance and assuming that 0.1% level difference between the two measurements can be detected, the minimum detectable SRI is of the order of 4×10⁻⁵. It is important to note that the first resonance on the short wavelength side of the core mode Bragg reflection is a ghost mode which does not appear in simulations unless a UV induced break in symmetry or relatively large core index increase is included in the calculation. This ghost mode may be useful in detection of fiber bends and shear stresses.

As mentioned above, it is desirable that the wavelength shifts measured in SRI sensing do not depend on the temperature of the sensor. The experiments show that while individual resonances of core and selected cladding modes each drift by ˜10 pm/° C. (for the first resonance (LP₀₁: Bragg wavelength resonance), the second resonance (ghost mode resonance) and the seventeenth resonance), as shown in FIG. 4, the relative wavelength shift between the cladding modes and the Bragg wavelength is less than 0.4 pm/° C. This represents a ˜27 times reduction in sensitivity from the resonance wavelengths themselves.

This confirms the usefulness of the self-referencing feature for making this SRI sensor temperature-independent.

Finally, the experimental results show that the TFBG spectrum changes as a function of bending in FIG. 5 (using a stronger grating written in hydrogen-loaded fiber). As expected since the fiber core lies on a neutral strain axis for pure bending, the core mode resonance and high order cladding modes remain unchanged but the first few low order cladding mode resonances change by several dB for the bending radii tested. Therefore, a single sensor can be used to detect three different physical manifestations simply by interrogating it over three different wavelength ranges: 1) near the Bragg resonance for temperature; 2) near low order cladding modes for bending; 3) at the short wavelength end of the transmission spectrum for SRI.

A particularly simple interrogation scheme for the TFBG could be constructed by spectrally slicing the transmitted light using an arrayed waveguide grating (AWG) butt-coupled to a detector array. The spacing and widths of the resonances are quite compatible with standard 40 or 80 channels AWGs that are widely available from telecommunications component vendors.

Instead of tracking wavelength shifts, the detectors monitor power level changes at fixed wavelength positions. One channel of the AWG interrogator can be used in closed loop to thermally tune the AWG (forcing its wavelength comb to follow the core mode resonance) while the output of another channel would reveal the power level changes associated with the relative wavelength shift of a cladding mode resonance. Level discretization and digital processing can then be used to extract meaningful data from the TFBG response. For more information on this interrogation scheme, the reader is invited to refer to the article of G. Z. Xiao, P. Zhao, F. G. Sun, Z. G. Lu, Z. Zhang, and C. P. Grover, “Interrogating fiber Bragg grating sensors by thermally scanning a demultiplexer based on arrayed waveguide gratings,” Opt. Lett., vol. 29, pp. 2222-2224, 2004. The content of this article is incorporated herein by reference.

Another scheme, requiring only a power detector, may be used to interrogate a selected cladding mode resonance independently of temperature or strain. This involves a pair of gratings as shown in FIG. 6. The TFBG is placed at the sensing point and is followed by a high reflectivity regular FBG of the same length (hence having the same spectral bandwidth) but with a Bragg wavelength AB centered near anyone of the cladding mode resonances (referred to as λC). Incident broadband light goes through the TFBG and gets attenuated at λC, continues to the FBG, gets reflected and returns towards after having passed a second time through the attenuating TFBG. This reflected light is recuperated from the fiber using either a circulator or a 3 dB coupler. The spectral bandwidth of the input light needs only to be large enough to cover the maximum wavelength excursion of the sensor in operation. The total amount of light reaching the detector consists of the residual Bragg reflection from the TFBG (which can be minimized for certain tilt angles) and the light reflected at λB. When λC=λB, very little light reaches the detector but as soon as the cladding mode resonance shifts or changes its strength the power level at the detector changes. As an example, such a pair of gratings were fabricated with λC>λB initially and then the FBG was stretched (thereby increasing λB) through the cladding mode resonance. Light from a pumped erbium-doped fiber broadband source was launched into the fiber containing the two gratings through a 3 dB coupler. The reflected power detected near the input of the fiber as a function of strain on the FBG is shown in FIG. 7. The graph shows that the power changes by more than 10 dB when λB becomes equal to λC. This represents a simple scheme to detect changes in Bragg wavelength or coupling strength without the need for an optical spectrum analyser. As shown in FIG. 4, if the two gratings illustrated in FIG. 6 are at the same temperature then λC and λB will move together and no power level change will be observed in the reflected signal. In most practical applications it would be the FBG that would provide the reference and the power level fluctuations would reflect changes in the TFBG cladding modes due to external perturbations. However the scheme is symmetrical with respect to the two gratings and the experimental results discussed here show that the sensitivity can be quite significant (0.1 dB/μStrain).

In a possible variant, the TFBG is designed to produce plasmon resonances to perform measurements on physical manifestations. A typical setup is shown in FIG. 9. The TFBG 900 is fabricated in an optical fiber 902. The cladding 904 of the optical fiber 902 is coated with a suitable metal layer 906, such as gold to produce a dielectric/conductor interface 910. Such dielectric/conductor interface 910 gives rise to surface plasmon resonances. Specifically, in order for plasmon resonances to occur, the optical field within the fiber 802 has to have non zero amplitude at the dielectric/conductor interface 910 and a specific value of axial propagation constant (or, if one considers the ray optics approach to wave guiding, of angle of incidence within the total internal reflection regime).

There are many potential applications for such structures, especially in the optical sensing of chemicals, among others. The TFBG 900 is used to couple the core mode light to a multitude of cladding modes, depending on the light wavelength, as shown in FIG. 8. The cladding modes have non-zero evanescent fields extending outside the cladding diameter and hence into the metal layer 906. When the axial component of the propagation constant of the cladding mode equals that of an SPR wave, coupling to that SPR wave can occur. A single TFBG is sufficient to generate a wavelength dependent set of cladding modes that are “interrogating” the metal layer 906 at various angles of incidence. This is most easily seen from the phenomenological representation shown in FIG. 9. FIG. 9 illustrates the ray optic analogy of the coupling from a guided core mode to several cladding modes through a TFBG. Each of the modes can be individually “addressed” simply by changing the wavelength of the guided light, and each mode strikes the cladding boundary at a different angle of incidence. On the other hand, FIG. 10 shows the traditional attenuated total reflection method (usually referred to as the Kretschmann configuration) to excite and detect SPR waves by changing the angle of incidence of the light beam incident on the metal film.

If there are non-radiative SPR waves that can be guided by the metal film surrounded on one side by silica glass and on the other side by a suitable medium, and if these SPR waves have an effective index (along the axis of the fiber) that is phase matched to one of the cladding modes effective indices, then coupling can occur between this cladding mode and the SPR wave. When this occurs, the cladding modes involved will experience more loss than their neighbours. The effective index of the i^(th) cladding mode (n^(i) _(clad)), can be calculated from the resonance position λ^(i) _(clad) by the following expression:

λ^(i) _(clad)=(n ^(i) _(eff) +n ^(i) _(clad))Λ/(cos θ)  (1)

where n^(i) _(eff) is the effective index of the core mode at λ^(i) _(clad), and Λ and θ are the period and the internal tilt angle of the TFBG. The wavelengths of the cladding mode resonances that are perturbed as a result of a coupling to a SPR wave in a metal-coated TFBG, such as the TFBG 900, the provide a direct measure of the effective index of the SPW through Equation (1).

In the course of experimental work fiber gratings were fabricated using the standard process of KrF excimer laser irradiation of hydrogen-loaded CORNING SMF28 fiber through a phase mask. The required tilt was achieved by rotating the mask-fiber assembly around an axis perpendicular to the fiber axis and to the plane of incidence of the laser light. The transmission spectrum of the grating used for the experiments reported is shown in FIG. 8. The longest wavelength resonance corresponds to the reflection of the core mode light onto itself (Bragg wavelength), while all the shorter wavelength resonances correspond to the excitation of backward propagating cladding modes. These modes are not reflected back to the source because they are rapidly attenuated by the fiber jacket as soon as they leave the grating region (where the jacket has been removed prior to fabricating the grating). For resonances between 1520 and 1560 nm, phase mask periods of the order of 1 μm are used. After fabrication, the gratings were heat-stabilized by subjecting them to a rapid annealing at −300° C. and the remaining hydrogen removed by 12 hours of heating at 120° C. prior to gold deposition. In these preliminary experiments, we used a small-scale sputtering chamber (Polaron Instruments model E5100) with the fiber positioned a few cm from the gold target. For flat samples in the same geometry, a gold layer thickness of 20 nm requires 1 minute of deposition at a pressure of 0.1 Torr, a potential difference of 2.5 kV, and 18-20 mA of sputter current. In order to coat the fiber as uniformly as possible, two coating runs were made with the fiber holder rotated by 180 degrees between the coatings. Under these conditions, the film uniformity around the fiber circumference is unlikely to be optimal. The film thickness on the fiber that is indicated in this specification is the value expected for the two sides of the fiber that directly facing the sputtering target during the two coating runs. While thicknesses ranging from 10 to 50 nm were tested, the following description will focus on results obtained with a 20 nm-thick nominal gold layer.

After the gold deposition, the fiber transmission spectrum is visibly modified, but without measurable features of interest, as shown in FIG. 11, indicating that the very thin gold layer has had an effect. However, no narrowband SPR resonances can be seen in the wavelength spectrum sampled by the cladding modes. When the gold-coated grating is immersed in liquids with various refractive indices (sugar solutions), anomalous resonances appear for certain very specific sugar concentrations, as determined from Abbe refractometer measurements of the refractive index of the solutions at 589 nm (nD) (FIG. 12). The accuracy of the Abbe refractometer that was used is of ±0.0002. These resonances are not the same as those obtained for uncoated tilted fiber gratings, since they have a finite bandwidth within the cladding mode envelope and their maximum attenuation shifts rapidly with the external index. The peak position of the anomalous resonance (λ_(p)) is obtained by fitting the envelope of the cladding mode resonances. FIG. 13 shows how λ_(p) changes as the refractive index of the outer medium is increased by small amounts. The spatial width of the envelope of the anomalous resonances is about 5 nm.

By using equation (1) to find the effective indices of the cladding modes within a resonance and the refractive index of silica near 1550 nm (n=1.444), it is possible to calculate the angular spread of the equivalent angles of incidences (since the effective index is equal to the projection on the fiber axis of the refractive index in silica). For the data of FIG. 12, the angular spread is 3.5 degrees (around a mean incidence angle θ=78°). This angle of incidence agrees with the predicted value for gold-coated silica glass in sucrose solutions interrogated at wavelengths close to 1500 nm. The angular spread of the resonance also corresponds well to typical values obtained for SPR measurements made with the Kretschmann configuration. Furthermore, the wavelength shift as a function of n_(D) is well approximated by a straight line with a slope of 454 pm/(10⁻³ change in n_(D)). Even considering the dispersion of the sugar solutions between 589 nm and the 1520-1560 nm region, this is again in quantitative agreement with the expected behavior for contra-directional gratings in gold-coated silica fibers where shifts of the order of 100-500 pm/(10⁻³ change in n_(ext)) were theoretically predicted. These observations support the hypothesis that the resonance seen is indeed due to a SPR that is perturbing some of the cladding modes. In particular, the effective indices of the plasmons that are observed are smaller than the glass refractive index but larger than the effective indices of the outer medium. This corresponds to a situation where the plasmons are seen as perturbed cladding modes with a local electromagnetic field maximum at the outer metal boundary. It is this local field maximum that enhances the sensitivity of the cladding mode resonance to the exact value of external index.

The SPR waves can be used for chemical and biological monitoring through changes in the refractive index of the medium in which the fiber is located or through changes in the refractive index of the gold layer itself.

The tilted grating discussed above can be used for a number of different sensing applications, examples of which are discussed below:

1. Strain Gage with Thermal Compensation

-   -   As discussed earlier, the response of the tilted grating         includes a core mode resonance component and a cladding mode         resonance component. The core mode resonance component conveys         the response of the sensor to temperature and elongation strain.         In a specific example as the temperature of the grating changes         or as elongation strain acts on the tilted grating the peak         wavelength of the core mode resonance will shift. On the other         hand, the wavelength gap between the peak wavelength of the core         mode resonance and anyone of the peaks in the cladding mode         resonance is generally constant with temperature. This means         that as the temperature changes this gap will also change.         However, if only elongation strain is applied on the sensor and         the temperature is maintained constant the peaks of the core         mode resonance component and of the cladding mode resonance         components will shift in unison maintaining the gap constant.         So, the gap change is indicative of the temperature variation         only. Once the wavelength shift due only to temperature is         determined, it suffices to subtract this wavelength shift from         say the total wavelength shift of the main peak of the core mode         resonance to determine the wavelength shift due only to         elongation strain. Once this wavelength shift is known, the         elongation strain can be derived easily.     -   FIG. 14 shows a measurement apparatus 1000 using a titled         grating. The measurement apparatus 1000 can be used to measure         elongation strain, independent of temperature variations. The         measurement is performed in a sensing zone 1002. Generally, the         measurement apparatus 1000 has an optical sensor 1004 which is         located in the sensing zone 1002 and includes a tilted grating,         a signal processing device 1006 which performs an analysis of         the optical response generated by the optical sensor 1004, and         an optical excitation generator 1008 that injects into the         optical sensor 1004 an optical excitation.     -   The sensing zone 1002 is the area where the measurement is to be         made. The optical sensor 1004 has a continuous length of optical         fiber. The optical fiber has a core in which is formed a TFBG.     -   In use, the optical excitation generator 1008 generates light         which is injected into the optical fiber length that leads to         the optical sensor 1004. The optical excitation reaches the TFBG         which filters out from the optical excitation wavelengths         corresponding to the peaks in the core and cladding mode         resonances. The optical excitation that reaches the signal         processing device 1006 is lacking the wavelengths filtered out         by the TFBG. The signal processing device 1006 uses the         information it receives from the optical sensor 1004 to derive         the intensity of the elongation strain acting on the optical         sensor 1004 in the sensing zone 1002, corrected for temperature         variations in the sensing zone 1002. If desired to measure         pressure or displacement acting on the optical sensor 1004,         there may be a necessity to mount the optical sensor 1004 on a         transducer structure (not shown in the drawings) that is         directly exposed to pressure or displacement and communicates         this pressure or displacement directly to the optical sensor         1004 in the form of elongation strain. Such transducer         structures are known in the art and do not need to be discussed         here in greater details.     -   Note that different measurement apparatus architectures can be         used. A variant is shown in FIG. 18, where the excitation and         the collection of the TFBG response are made from the same side         of the optical fiber. Specifically, in this embodiment, light         generated by the optical generator 1008 is injected into the         optical fiber length that leads to an optical coupler and to the         optical sensor 1004. The optical excitation reaches the TFBG         which filters out from the optical excitation wavelengths         corresponding to the peaks in the core and cladding mode         resonances. The optical fiber includes a reflector 1017 that         will reflect back toward the signal processing device/optical         excitation generator combination, the response produced by the         TFBG. The reflector 1017 can be formed at a termination point of         the optical fiber. In a specific example of implementation, the         termination is a straight termination. Different mechanisms can         be used to provide reflectivity at the reflector 1017. In one         example, the reflectivity is achieved via Fresnel reflection. In         a different example, the reflectivity is provided by a         reflective coating deposited on the straight termination. In         another possible example, the reflector 1017 is a non-tilted         grating that has a reflection spectrum wide enough to overlap         two or more of the cladding mode resonances produced by the         tilted grating.     -   Another possibility is to form on the sensor a coating which         expands in response to a certain substance or condition. The         expansion causes elongation strain which can then indirectly         either detect the substance or measure its concentration. The         specification provides below examples of substances that respond         to physical manifestations and that can induce a measurable         strain on the sensor.     -   The signal processing device extracts from the response from the         optical sensor 1004 information on the elongation strain acting         on the optical sensor 1004 along with the temperature in the         sensing zone 1002. FIG. 15 is a block diagram of the signal         processing device 1006. The signal processing device 1006 is         based on a computer platform that enables to perform digital         signal processing on the response received from the optical         sensor 1004 such as to derive the information desired. More         specifically, the signal processing device 1006 includes in         input interface 1010 that is coupled to the optical fiber length         leading directly to the optical sensor 1004. The input interface         1010 will convert the signal into an electric digital signal,         including performing appropriate filtering. The digital signal         is then impressed on the data bus 1012 that establishes a         communication path between a processor 1016 and a memory device         1014. The processor 1016 executes program code that processes         the data in the digital signal to extract information on the         elongation strain and temperature, according to the logic         discussed above.     -   The signal processing device 1006 also has an output interface         1018 that allows communicating the result of the mathematical         processing to an external entity. The external entity can be a         human operator or a piece of equipment that uses the information         generated by the signal processing device 1006 for specific         purposes.     -   Accordingly, the measurement apparatus 1000 can measure the         elongation strain acting on the optical sensor 1004 and the         temperature in the sensing zone 1002.

2. Multi-Purpose Sensor

-   -   As discussed previously, the TBFG can be used to sense two or         more physical manifestations. A sensor can be provided to         measure the index of refraction adjacent the sensing surface of         the sensor, and another physical manifestation such as         elongation strain and/or temperature. The measurement of the         elongation strain and/or temperature was discussed earlier. The         measurement of the index of refraction can be made in two         different ways. One is to track a wavelength shift of the         cladding mode resonances, as discussed in connection with FIGS.         2 a and 2 b. The other is to detect the occurrence of surface         plasmon resonances, by determining which one of the cladding         modes will experience loss, as discussed earlier.

3. Bending and Strain Gage and/or Temperature Sensor

-   -   As discussed earlier, the cladding mode resonances of the TFBG         can be used to detect bending, in particular the low order         cladding modes. Accordingly, by interrogating the TFBG in the         wavelength that corresponds to those low order cladding modes,         the degree of bending of the sensor can be determined. In a         specific example, the power ratio of the first two resonances in         the cladding mode varies with the degree of bending and can be         used to accurately measure this parameter. At the same time the         elongation strain and/or temperature can be measured by the         techniques discussed earlier. The measurement of the degree of         bending of the sensor can be used as an indirect measure of         another physical manifestation which acts on the sensor to         induce bending in it. For example, the sensor can be mounted on         a diaphragm that is exposed to pressure. The degree of pressure         determines the extent to which the diaphragm bows out. A sensor         placed on the diaphragm will be caused to bend accordingly, and         by measuring the degree of bending one can determine the extent         to which the diaphragm bows and consequently the pressure acting         on the diaphragm. This is shown in FIG. 16. The diaphragm 2000         that is made of flexible material is exposed to pressure on its         side 2002. A sensor 2004 is mounted to the diaphragm. The sensor         is placed on the convex side of the diaphragm 2000 but it can         also be placed on the concave side as well.     -   Another possibility is to coat the sensor with a substance that         induces bending in the sensor in response to a certain physical         manifestation. For example, the coating can be placed only on         one side of the sensor. The material of the coating is selected         such that it swells when in contact with the substance to         detect. When the coating swells it induces bending in the sensor         which can be detected as indicated earlier. By properly         selecting the coating the sensor can thus be made responsive to         a wide variety of substances, such as humidity (water), chemical         substances and biological substances.     -   In one example, the substances that would be responsive to water         are, but not limited to, water-swellable materials.     -   The water-swellable material relates, for example, to a         particulate absorbent material comprising a particulate core of         absorbent polymers, coated with a coating agent, comprising or         being an organic coating compound, which has one or more polar         groups.     -   The absorbent polymer refers, for example, to a polymer, which         is water-insoluble, water-swellable or gelling. These polymers         are typically lightly cross-linked polymers, which contain a         multiplicity of acid functional groups such as carboxylic acid         groups. Examples of acid polymers suitable for use herein         include those which are prepared from polymerizable,         acid-containing monomers, or monomers containing functional         groups which can be converted to acid groups after         polymerization. Thus, such monomers include olefinically         unsaturated carboxylic acids and anhydrides, and mixtures         thereof. The acid polymers can also comprise polymers that are         not prepared from olefinically unsaturated monomers.     -   Examples of such polymers include, but are not limited to,         polysaccharide-based polymers such as carboxymethyl starch and         carboxymethyl cellulose, and poly(amino acid) based polymers         such as poly(aspartic acid).     -   Some non-acid monomers can also be included, usually in minor         amounts, in preparing the absorbent polymers herein. Such         non-acid monomers can include, for example, but not limited to,         monomers containing the following types of functional groups:         carboxylate or sulfonate esters, hydroxyl groups, amide-groups,         amino groups, nitrile groups, quaternary ammonium salt groups,         and aryl groups (e.g., phenyl groups, such as those derived from         styrene monomer). Other optional non-acid monomers include         unsaturated hydrocarbons such as ethylene, propylene, 1-butene,         butadiene, and isoprene.     -   Olefinically unsaturated carboxylic acid and anhydride monomers         useful herein include the acrylic acids typified by acrylic acid         itself, methacrylic acid, alpha.-chloroacrylic acid,         a-cyanoacrylic acid, beta.-methylacrylic acid (crotonic acid),         .alpha.-phenylacrylic acid, .beta.-acryloxypropionic acid,         sorbic acid, .alpha.-chlorosorbic acid, angelic acid, cinnamic         acid, p-chlorocinnamic acid, .beta.-stearylacrylic acid,         itaconic acid, citroconic acid, mesaconic acid, glutaconic acid,         aconitic acid, maleic acid, fumaric acid, tricarboxyethylene,         and maleic anhydride.     -   In one specific example, the absorbent polymers contain carboxyl         groups. These polymers include hydrolyzed starch-acrylonitrile         graft copolymers, partially neutralized hydrolyzed         starch-acrylonitrile graft copolymers, starch-acrylic acid graft         copolymers, partially neutralized starch-acrylic acid graft         copolymers, hydrolyzed vinyl acetate-acrylic ester copolymers,         hydrolyzed acrylonitrile or acrylamide copolymers, slightly         network cross linked polymers of any of the foregoing         copolymers, polyacrylic acid, and slightly network cross linked         polymers of polyacrylic acid. These polymers can be used either         solely or in the form of a mixture of two or more different         polymers.     -   Those of skills in the art will be familiar with the process of         coating the water-swellable material onto the sensor.     -   In a further example, the water-swellable material includes, but         is not limited to, clay intimately mixed with a polypropene, a         polybutene or a mixture of polypropene and polybutene, and a         clay binding ion-exchange or coupling agent compound, to provide         a composition having an unexpected capacity for swelling upon         contact with water. The composition should include a clay binder         that is ion-exchanged with clay platelet cations on internal         negative charge sites of the clay platelets, or reacted with         hydroxyl moieties at the clay platelet edges to achieve         unexpected water-swellability.     -   The water-swellable clay utilized can be, but not limited to,         any water-swellable layered material, such as a smectite clay,         which will swell upon contact with water. The clay may be         smectite clay, such as a montmorillonite or a bentonite clay.         This clay has sodium as a predominant exchange cation. However,         the clay utilized to may also contain other cations such as         magnesium and iron.     -   Those skilled in the art will be familiar with the methods for         preparation of the compositions described herein.

4. Chemical/Biological Sensor

-   -   The sensor using a TFBG uses an interface responsive to the         biological or chemical element to be detected to produce a         physical manifestation that can be measured by the sensor. The         interface can be designed to impress on the sensor a physical         force which can be directly measured. An example of such         interface was mentioned earlier and it would typically be in the         form of a coating that causes the sensor to bend or stretch when         it comes in contact with the biological or chemical element to         be detected.     -   The interface can also be such as to cause SRI changes in         response to the presence of the biological or chemical element         to be detected. The chemical or biological sensor can also         function without the need of an interface detects SRI changes         which can be used in applications where a direct measure of the         SRI is required or applications where the SRI change is an         indicator of the occurrence of a chemical or a biological         process or element. As briefly mentioned above, the SRI can be         measured in two different manners. One involves tracking the         wavelength shift by of the cladding mode resonances, as         discussed in connection with FIGS. 2 (a) and 2 (b). The other         uses the detection of SPR.     -   The sensor can be used as a chemical sensor to detect changes in         SRI caused by the presence of a chemical element such that, but         not limited to, the sensor can be used for determining the         concentration of sugar in a medium, such as an aqueous solution,         for determining the concentration of alcohol in a medium, for         measuring the degree of curing of an adhesive, as the adhesive         cures the SRI changes. By measuring the SRI one can track the         degree of curing or detect a threshold at which the adhesive is         considered to be cured. The invention is also used for measuring         the degree of curing of cement in a fashion similar to the         curing of an adhesive. The invention is further used as a         biological detector. Generally, the biological detector includes         an interface.     -   As used herein, the term chemical or biological analyte refers         to a chemical or biological element to be detected.     -   The chemical and biological analytes that are contemplated         include, but are not limited to, bacteria; yeasts; fungi;         viruses; rheumatoid factor; antibodies, including, but not         limited to IgG, IgM, IgA and IgE antibodies; carcinoembryonic         antigen; streptococcus Group A antigen; antigen; viral antigens;         antigens associated with autoimmune disease; allergens; tumor         antigens; streptococcus Group B antigen, HIV I or HIV II         antigen; or host response (antibodies) to these and other         viruses; antigens specific to RSV or host response (antibodies)         to the virus; an antibody; antigen; enzyme; hormone;         polysaccharide; protein; prions; lipid; carbohydrate; drug;         nucleic acid; Salmonella species; Candida species, including,         but not limited to Candida albicans and Candida tropicalis;         Salmonella species; Neisseria meningitides groups A, B, C, Y and         W sub 135, Streptococcus pneumoniae; E. coli K1. E. coli;         Haemophilus influenza type B; an antigen derived from         microorganisms; a hapten; a drug of abuse; a therapeutic drug;         environmental agents; and antigens specific to Hepatitis; an         enzyme; a DNA fragment; an intact gene; a RNA fragment; a small         molecule; a metal; a toxin; a nucleic acid; a cytoplasm         component; pili or flagella component; or any other analytes.     -   The analyte of interest would typically be in a carrier medium         which can be solid, gel-like, liquid or gas. For instance         analyte can be detected in a bodily fluid such as mucous,         saliva, urine, fecal analyte, tissue, marrow, cerebral spinal         fluid, serum, plasma, whole blood, sputum, buffered solutions,         extracted solutions, semen, uro-genital secretion, pericardial,         gastric, peritoneal, pleural, throat swab, subfractions thereof,         or other washes and the like. A gaseous medium may be, but not         limited to, air.     -   An aqueous buffered solution may be employed to dilute the         analyte, such an aqueous buffer solution may be lightly or         heavily buffered depending on the nature of the analyte to be         detected. Various buffers may be employed such as carbonate,         phosphate, borate, Tris, acetate, barbital, Hepes, or the like.         Organic polar solvents, e.g., oxygenated neutral solvents, may         be present in amounts ranging from about 0 to 40 volume percent         such as methanol, ethanol, .alpha.-propanol, acetone,         diethylether, or the like.     -   In one specific example, the interface has a recognition analyte         that can be attached on the sensing surface of the sensor and         will associate with the analyte of interest (FIG. 1). When such         binding occurs the SRI changes. The change is specific in that         the SRI acquires a specific value which indicates that the         binding has taken place. The cladding mode resonances         “interrogate” the electric/dielectric interface and when the SRI         acquires the specific value, the energy in at least one of the         resonance will be transferred into a SPR. The energy transfer         will show as a reduced power in that particular cladding mode         resonance, allowing determining that the SRI is at the specific         value indicative of a binding event. Note that since several         resonances interrogate the electric/dielectric interface, this         technique can at least in theory monitor simultaneously for a         set of different specific SRI values, each associated to a given         resonance in the cladding mode resonance component. As such,         several binding events, corresponding to different SRI values,         can be detected simultaneously.     -   The recognition analyte is thus a component of a specific         binding pair and includes, but is not limited to,         antigen/antibody, enzyme/substrate, oligonucleotide/DNA,         chelator/metal, enzyme/inhibitor, bacteria/receptor,         virus/receptor, hormone/receptor, DNA/RNA, or RNA/RNA,         oligonucleotide/RNA, and binding of these species to any other         species, as well as the interaction of these species with         inorganic species.     -   The recognition analyte that is attached to the sensing surface         can be, but is not limited to, toxins, antibodies, antigens,         hormone receptors, parasites, cells, haptens, metabolizers,         allergens, nucleic acids, nuclear analytes, autoantibodies,         blood proteins, cellular debris, enzymes, tissue proteins,         enzyme substrates, coenzymes, neuron transmitters, viruses,         viral particles, microorganisms, proteins, polysaccharides,         chelators, drugs, and any other member of a specific binding         pair. The recognition analyte is specifically designed to         associate with the analyte of interest.     -   The recognition analyte may be passively adhered to the sensing         surface. If required, the recognition analyte may be covalently         attached to the sensing surface of the sensor. The chemistry for         attachment of recognition analyte is well known to those skilled         in the art.     -   Recognition analyte for detection of bacteria may have binding         activity to specifically bind a surface membrane component,         protein or lipid, a polysaccharide, a nucleic acid, or an         enzyme. The analyte, which is specific to the bacteria, may be a         polysaccharide, an enzyme, a nucleic acid, a membrane component,         or an antibody produced by the host in response to the bacteria.         The presence of the analyte may indicate an infectious disease         (bacterial or viral), cancer or other metabolic disorder or         condition. The presence of the analyte may be an indication of         food poisoning or other toxic exposure. The presence of the         analyte may also be an indication of bacterial contamination.     -   A wide range of techniques can be used to apply the recognition         analyte of the interface to the sensing surface of the sensor.         The sensing surface may be, for example but not limited to,         coated with recognition analyte by total immersion in a solution         for a predetermined period of time; application of solution in         discrete arrays or patterns; spraying, ink jet, or other         imprinting methods; or by spin coating from an appropriate         solvent system. The technique selected should minimize the         amount of recognition analyte required for coating a large         number of test surfaces and maintain the stability/functionality         of recognition analyte during application.

Attachment of the Recognition Analyte by Self-Assembled Monolayers on Sensing Surface

-   -   In a further embodiment, the invention includes attachment of         the recognition analyte into the surface or the interior of the         sensing surface, through self-assembled monolayers.     -   Self-assembled monolayers can be prepared using different types         of molecules and different substrates. Commonly used examples         are, but not limited to, alkylsiloxane monolayers, fatty acids         on oxidic materials and alkanethiolate monolayers. This type of         self-assembled monolayers holds great promise for applications         in several different areas. Some examples of suggested and         implemented applications are, but not limited to, molecular         recognition, self-assembly monolayers as model substrates and         biomembrane mimetics in studies of biomolecules at surfaces,         selective binding of enzymes to surfaces.     -   There are many different systems of self-assembling monolayers         based on different organic components and supports, such as, but         not limited to, systems of alkanethiolates, HS(CH.sub.2).sub.n         R, on gold layers. The alkanethiols chemisorb on the gold         surface from a solution in which the gold layer is immersed, and         form adsorbed alkanethiolates with loss of hydrogen. Adsorption         can also occur from the vapor. Self-assembling monolayers formed         on gold from long-chain alkanethiolates of structure         X(CH.sub.2).sub.n Y(CH.sub.2).sub.m S are highly ordered. A wide         variety of organic functional groups (X,Y) can be incorporated         into the surface or interior of the monolayer.     -   In one example, the self-assembling monolayer is formed of a         carboxy-terminated alkane thiol stamped with a patterned         elastomeric stamp onto a gold surface. The alkanethiol is inked         with a solution of alkanethiol in ethanol, dried, and brought         into contact with a surface of gold. The alkanethiol is         transferred to the surface only at those regions where the stamp         contacts the surface, producing a pattern of self-assembling         monolayer which is defined by the pattern of the stamp.         Optionally, areas of unmodified gold surface next to the stamped         areas can be rendered hydrophobic by reaction with a         methyl-terminated alkane thiol. The details of the method are         well known in the art.     -   The present invention, the self-assembling monolayer has the         following general formula: X is reactive with metal or metal         oxide. For example, X may be asymmetrical or symmetrical         disulfide (—R′SSY′, —RSSY), sulfide (—R′SY′, —RSY), diselenide         (—R′Se—SeY′), selenide (—R′SeY′, —RSeY), thiol (—SH), nitrile         (—CN), isonitrile, nitro (—NO.sub.2), selenol (—SeH), trivalent         phosphorous compounds, isothiocyanate, xanthate, thiocarbamate,         phosphine, thioacid or dithioacid, carboxylic acids, hydroxylic         acids, and hydroxamic acids.     -   R and R′ are hydrocarbon chains which may optionally be         interrupted by hetero atoms and which are preferably         non-branched for the sake of optimum dense packing. At room         temperature, R is greater than or equal to seven carbon atoms in         length, in order to overcome natural randomizing of the         self-assembling monolayer. At colder temperatures, R may be         shorter. In one embodiment, R is —(CH.sub.2).sub.n- where n is         between 10 and 12, inclusive. The carbon chain may optionally be         perfluorinated. A person skilled in the art will understand that         the carbon chain may be of any length.     -   Y and Y′ may have any surface property of interest. For example,         Y and Y′ could be any among the great number of groups used for         immobilization in liquid chromatography techniques, such as         hydroxy, carboxyl, amino, aldehyde, hydrazide, carbonyl, epoxy,         or vinyl groups.     -   Self-assembling monolayers of alkyl phosphonic, hydroxamic, and         carboxylic acids may also be useful for the methods of the         present invention. Since alkanethiols do not adsorb to the         surfaces of many metal oxides, carboxylic acids, phosphonic         acids, and hydroxamic acids may be preferred for X for those         metal oxides.     -   R may also be of the form (CH.sub.2).sub.a-Z-(CH.sub.2).sub.b,         where a.gtoreq.0, B.gtoreq.7, and Z is any chemical         functionality of interest, such as sulfones, urea, lactam, and         the like.     -   The stamp may be applied in air, gel, semi-gel, or under a         fluid, such as water to prevent excess diffusion of the         alkanethiol. For large-scale or continuous printing processes,         it is most desirable to print in air, since shorter contact         times are desirable for those processes.     -   In one specific example the sensor can be used in immunoassay         methods for either antigen or antibody detection. The sensors         may be adapted for use in direct, indirect, or competitive         detection schemes, for determination of enzymatic activity, and         for detection of small organic molecules such as, but not         limited to drugs of abuse, therapeutic drugs, environmental         agents), as well as detection of nucleic acids and         microorganisms.     -   For immunoassays, an antibody may serve as the recognition         analyte or it may be the analyte of interest. The recognition         analyte, for example an antibody or an antigen, should form a         stable, dense, reactive layer on the attachment layer of the         test sensor. If an antigen is to be detected and an antibody is         the recognition analyte, the antibody should be specific to the         antigen of interest; and the antibody (recognition analyte)         should bind the antigen (analyte) with sufficient avidity that         the antigen is retained at the surface of the sensing surface.         In some cases, the analyte may not simply bind the recognition         analyte, but may cause a detectable modification of the         recognition analyte to occur. This interaction could cause an         increase in mass at the test surface or a decrease in the amount         of recognition analyte on the test surface. An example of the         latter is the interaction of a degradative enzyme or analyte         with a specific, immobilized substrate. In this case, one would         see a diffraction pattern before interaction with the analyte of         interest, but the diffraction pattern would disappear if the         analyte were present.     -   In another specific example, the sensor applies to detection of         nucleic acid molecules and nucleic acid probes. Nucleic acids         can be attached to sensing surfaces in an hybridization assays.         A sensing surface such as gold is modified with nucleic acids         via for example, but not limited to, linkers, and blocking         moieties, which serve to shield the nucleic acids from the         sensing surface.     -   “Blocking moieties” are molecules which are attached to the         sensing solid support and function to shield the nucleic acids         from the sensing surface. For the purposes of this invention,         the attachment of a sulfur moiety to a sensing surface, such as         gold, is considered covalent.     -   By “nucleic acids” or “oligonucleotides” herein is meant at         least two nucleotides covalently linked together. A nucleic acid         of the present invention will generally contain phosphodiester         bonds, although in some cases, as outlined below, a nucleic acid         analogs are included that may have alternate backbones,         comprising, for example, phosphoramide, phosphorothioate,         phosphorodithioate, O-methylphosphoroamidite linkages and         peptide nucleic acid backbones and linkages.     -   The nucleic acids of the invention may also be characterized as         “probe” nucleic acids and “target” nucleic acids. These terms         are known in the art. Either probe or target nucleic acids may         be attached to the solid support via linker. In a preferred         embodiment, the probe nucleic acids are attached, via linker         moieties, to the solid support, and the target nucleic acids are         added in solution. The nucleic acid and the probe may be         labeled.     -   Probe nucleic acids or probe sequences are preferably single         stranded nucleic acids. The probes of the present invention are         designed to be complementary to the target sequence, such that         hybridization of the target sequence and the probes of the         present invention occur. This complementarity need not be         perfect; there may be any number of base pair mismatches which         will interfere with hybridization between the target sequence         and the single stranded probe nucleic acids of the present         invention. However, if the number of mutations is so great that         no hybridization can occur under even the least stringent of         hybridization conditions, the sequence is not a complementary         target sequence.     -   It will be appreciated by those in the art, the length of the         probe will vary with the length of the target sequence and the         hybridization and wash conditions.     -   In a possible variant, the nucleic acid is attached to the         sensing surface in monolayers. The techniques to attach nucleic         acid molecules to the sensing surface will be well known to         those skilled in the art.     -   “Target nucleic acids” or “sequences” means a nucleic acid         sequence on a single strand of nucleic acid. The target sequence         may be a portion of a gene, a regulatory sequence, genomic DNA,         cDNA, mRNA, or others. It may be any length, with the         understanding that longer sequences are more specific. As is         outlined herein, probes are made to hybridize to target         sequences to determine the presence or absence of the target         sequence in a sample. Target nucleic acids may be prepared or         amplified as is commonly known in the art. When target nucleic         acids are attached to the sensing surface, they will generally         be the same size as outlined for probe nucleic acids, above.     -   In general, blocking moieties have at least a first and a second         end. The first end is used to covalently attach the blocking         moiety to the sensing surface. The second end terminates in a         terminal group, defined below. However, in some embodiments, the         blocking moieties may be branched molecules. Thus, for example,         the first end is used for attachment to the solid support and         all or some of the other ends may terminate in a terminal group,         as defined below.     -   The second end of the blocking moiety terminates in a terminal         group. By “terminal group” or “terminal moiety” herein is meant         a chemical group at the terminus of the blocking moiety. The         terminal groups may be chosen to modulate the interaction         between the nucleic acid and the blocking moieties, or the         surface. Thus, for example, in another embodiment, when the         blocking moieties form a monolayer as is generally described         below, the terminal group may be used to influence the exposed         surface of the monolayer. Thus, for example, the terminal group         may be neutral, charged, or sterically bulky. For example, the         terminal groups may be negatively charged groups, effectively         forming a negatively charged surface such that when the probe or         target nucleic acid is DNA or RNA the nucleic acid is repelled         or prevented from lying down on the surface, to facilitate         hybridization. This may be particularly useful when the nucleic         acid attached to the sensing surface via a linker moiety is         long.     -   In addition to the blocking moieties, the sensing surface of the         invention comprises modified nucleic acids.     -   The nucleic acids of the invention are modified with linker         moieties, to form modified nucleic acids which are attached to         the sensing surface. By “modified nucleic acid” herein is meant         a nucleic acid as defined above covalently attached to a linker         moiety.     -   By “linker moieties” is meant molecules which serve to         immobilize the nucleic acid at a distance from the sensing         surface. Linker moieties have a first and a second end. The         first end is used to covalently attach the linker moiety to the         sensing surface. The second end is used for attachment to the         nucleic acid.     -   The blocking moieties are made using techniques well known in         the art.     -   In a further variant, the present invention is useful in methods         of assaying for the presence or absence of target nucleic acids         in the sample to be analyzed. Thus, the present invention         provides methods of hybridizing probe nucleic acids to target         nucleic acids. The methods comprise adding or contacting target         nucleic acids to a sensing surface of the invention. The sensing         surface comprises blocking moieties, and modified probe nucleic         acids. The contacting is done under conditions where the probe         and target nucleic acids, if suitably complementary, will         hybridize to form a double-stranded hybridization complex.     -   The assay conditions may vary, as will be appreciated by those         in the art, and include high, moderate or low stringency         conditions as is known in the art. The assays may be done at a         variety of temperatures, and using a variety of incubation         times, as will be appreciated by those in the art. In addition,         a variety of other reagents may be included in the hybridization         assay, including buffers, salts, proteins, detergents or the         like. Positive and negative controls are generally run.     -   In a further specific example, the sensor is used to detect         microorganisms in medium.     -   The term “microorganism” as used herein means an organism too         small to be observed with the unaided eye and includes, but is         not limited to bacteria, a cell, cells, viruses, protozoans,         fungi, and ciliates.     -   In another embodiment, microorganisms such as, but not limited         to, bacteria, bacteriophages, viruses, and cellular analyte can         be detected by sampling the nucleic acids, such as, but not         limited to, nucleotides or polynucleotides that they contain or         release. Microorganisms can also be detected by sampling the         protein, carbohydrates and/or lipids that they contain or         release.     -   In a possible variant, the microorganims may be detected by a         recognition analyte such as not limited to an antibody assembled         as a monolayer on the sensing surface or may be detected by         interacting directly on the bare sensing surface.     -   The embodiments of the invention described herein can be used in         several application areas, for example, but not limited to, for         the quantitative or qualitative determination of chemical,         biochemical or biological analytes in screening assays in         pharmacological research, for real-time binding studies or in         the determination of kinetic parameters in affinity screening or         in research, for DNA and RNA analytics and for the determination         of genomic or proteomic differences in the genome, for the         determination of protein-DNA interactions, for the determination         of regulation mechanisms for mRNA expression and protein         (bio)synthesis, for the determination of biological or chemical         markers, such as mRNA, proteins, peptides or low molecular         organic (messenger) compounds, for the determination of         antigens, pathogens or bacteria in pharmacological product         research and development, for therapeutic drug selection, for         the determination of pathogens, harmful compounds or germs, such         as, but not limited to, salmonella, prions, viruses and         bacteria.     -   Although various embodiments have been illustrated, this was for         the purpose of describing, but not limiting, the invention.         Various modifications will become apparent to those skilled in         the art and are within the scope of this invention, which is         defined more particularly by the attached claims. 

1) A sensor for sensing at least one physical manifestation occurring in a medium, said sensor comprising: a) a sensing surface for exposure to the medium; b) an optical pathway; c) a tilted grating in said optical pathway, said grating being responsive to electromagnetic radiation propagating in said optical pathway to generate a response conveying information on the at least one physical manifestation. 2) A sensor as defined in claim 1, wherein said grating induces SPR in proximity to said sensing surface. 3) A sensor as defined in claim 2, wherein said sensing surface includes metallic material. 4) A sensor as defined in claim 3, wherein said metallic material is in the form of a coating over said optical pathway. 5) A sensor as defined in claim 4, wherein said optical pathway and said metallic coating form a dielectric/electrical conductor interface. 6) A sensor as defined in claim 5, wherein said metallic coating is homogenous. 7) A sensor as defined in claim 5, wherein said metallic coating is heterogeneous. 8) A sensor as defined in claim 5, wherein said optical path includes an optical fiber. 9) A sensor as defined in claim 5, wherein said metallic coating includes gold material. 10) A sensor as defined in claim 1, wherein said physical manifestation is an SRI change. 11) A sensor as defined in claim 10, wherein said sensor is a biosensor. 12) A sensor as defined in claim 11, wherein said biosensor is responsive to bacteria. 13) A sensor as defined in claim 11, wherein said biosensor is responsive to a virus. 14) A sensor as defined in claim 10, wherein said sensor is responsive to one or more chemical compounds. 15) A sensor as defined in claim 10, wherein said sensor is an alcohol sensor. 16) A sensor as defined in claim 10, wherein said sensor is a sugar sensor. 17) A sensor as defined in claim 10, wherein said sensor measures a rate of curing of a material. 18) A sensor as defined in claim 17, wherein the material is adhesive. 19) A sensor as defined in claim 17, wherein the material is cement. 20) A sensor as defined in claim 10, wherein said sensor measures SRI in the range from about 1.29 to about 1.45. 21) A sensor as defined in claim 10, wherein the response includes at least two components. 22) A sensor as defined in claim 10, wherein the response includes a core mode resonance component. 23) A sensor as defined in claim 22, wherein the response further including a cladding mode resonance component. 24) A sensor for sensing a physical manifestation occurring externally of said sensor, said sensor comprising: a) a sensing surface for exposure to the physical manifestaton; b) an optical pathway; c) a tilted grating in said optical pathway, said tilted grating being responsive to electromagnetic radiation propagating in said optical pathway to induce SPR adjacent said sensing surface. 25) A sensor as defined in claim 24, wherein the physical manifestation is an SRI change. 26) A sensor as defined in claim 25, wherein said sensing surface has a metallic material. 27) A sensor as defined in claim 26, wherein said metallic material is in the form of a coating. 28) A sensor as defined in claim 27, wherein said coating and said optical pathway form an electrical conductor/dielectric interface. 29) A sensor as defined in claim 28, wherein said sensor is a biosensor. 30) A sensor as defined in claim 29, wherein said sensor is a microorganism sensor. 31) A sensor as defined in claim 29, wherein said sensor is a bacterial sensor. 32) A sensor as defined in claim 28, wherein said sensor measures concentration of alcohol. 33) A sensor as defined in claim 28, wherein said sensor measures concentration of sugar. 34) A sensor as defined in claim 28, wherein said sensor is a chemical compound sensor. 35) A sensor as defined in claim 28, wherein said sensor measures rate of curing of a material. 36) A sensor as defined in claim 35, wherein the material is adhesive. 37) A sensor as defined in claim 35, wherein the material is cement. 38) A method for detecting the presence of bacteria in a medium, comprising: a) providing a sensor having a sensing surface, said sensor having an optical pathway containing a tilted grating; b) placing the sensing surface in contact with the medium; c) determining from a response of said sensor if the bacteria is present in the medium. 39) A method for detecting the presence of virus in a medium, comprising: a) providing a sensor having a sensing surface, said sensor having an optical pathway containing a tilted grating; b) placing the sensing surface in contact with the medium; c) determining from a response of said sensor if the virus is present in the medium. 40) A method for measuring the concentration of sugar in a medium, comprising: a) providing a sensor having a sensing surface, said sensor having an optical pathway containing a tilted grating; b) placing the sensing surface in contact with the medium; c) determining from a response of said sensor the concentration of sugar in the medium. 41) A method for measuring the concentration of alcohol in a medium, comprising: a) providing a sensor having a sensing surface, said sensor having an optical pathway containing a tilted grating; b) placing the sensing surface in contact with the medium; c) determining from a response of said sensor the concentration of alcohol in the medium. 42) A method for measuring a degree of curing of a curable material, comprising: a) providing a sensor having a sensing surface, said sensor having an optical pathway containing a tilted grating; b) placing the sensing surface in contact with the curable material; c) determining from a response of said sensor the degree of curing of the curable material. 43) A method as defined in claim 42, wherein the material is an adhesive. 44) A method as defined in claim 42, wherein the material is cement. 45) A sensor as defined in claim 1, wherein the response of said grating includes at least two components distinguishable from one another. 46) A sensor as defined in claim 45, wherein one component of the response is distinguishable from another component of the response on the basis of wavelength. 47) A sensor as defined in claim 45, wherein one component of the response is a cladding mode resonance component. 48) A sensor as defined in claim 47, wherein another component of the response is a core mode resonances component. 49) A sensor as defined in claim 45, wherein the components convey information allowing distinguishing reaction of the sensor to temperature from reaction of the sensor to a physical manifestation other than temperature. 50) A sensor as defined in claim 49, wherein the physical manifestation other than temperature is SRI. 51) A sensor as defined in claim 49, wherein the physical manifestation other than temperature is elongation strain. 52) A sensor as defined in claim 49, wherein the physical manifestation other than temperature is bending. 53) A sensor as defined in claim 49, wherein one component conveys the reaction of the sensor to temperature. 54) A sensor as defined in claim 53, wherein the component conveying the reaction of the sensor to temperature is a core mode resonance component. 55) A sensor as defined in claim 49, wherein one component conveys the reaction of the sensor to bending. 56) A sensor as defined in claim 55, wherein the one component that conveys the reaction of the sensor to bending is a cladding mode resonance component. 57) A sensor as defined in claim 49, wherein one component conveys the reaction of the sensor to SRI change. 58) A sensor as defined in claim 57, wherein the one component that conveys the reaction of the sensor to the SRI change is a cladding mode resonance component. 59) A sensor as defined in claim 49, wherein said sensor includes an optical fiber establishing said optical pathway. 60) Elongation strain sensor, comprising: a) An optical pathway; b) A tilted grating in said optical pathway to generate a response conveying information on elongation strain acting on said sensor. 61) Elongation strain sensor as defined in claim 60 wherein said optical pathway includes an optical fiber. 62) Elongation strain sensor as defined in claim 60, wherein the response includes a core mode resonance component and a cladding mode resonances component. 63) Method for measuring elongation strain, comprising: a) receiving a response from a sensor containing a tilted grating subjected to elongation strain, the response conveying information on: i) reaction of the tilted grating to elongation strain; ii) reaction of the tilted grating to temperature; b) processing the response of the tilted grating to distinguish the reaction of the tilted grating to elongation strain from the reaction of the tilted grating to temperature. 64) Apparatus for measuring elongation strain, compressing: a) an elongation strain sensor, having: i) an optical pathway; ii) a tilted grating in said optical pathway to generate a response conveying information on elongation strain and temperature acting on said sensor; b) a signal processing unit to process the response of the tilted grating and distinguish in the response to reaction of said tilted grating to elongation strain from the reaction of the tilted grating to temperature. 65) Apparatus as defined in claim 64 wherein said optical pathway includes an optical fiber. 66) Apparatus as defined in claim in claim 64, wherein the response of said tilted grating includes a core mode resonance component and a cladding mode resonances component. 67) Bending strain sensor, comprising: a) an optical pathway; b) A tilted grating in said optical pathway to generate a response conveying information on bending strain acting on said sensor. 68) Bending strain sensor as defined in claim 67 wherein said optical pathway includes an optical fiber. 69) Bending strain sensor as defined in claim 67, wherein the response includes a core mode resonance component and a cladding mode resonances component. 70) Method for measuring bending strain, comprising: a) receiving a response from a sensor containing a tilted grating subjected to bending strain, the response conveying information on: i) reaction of the tilted grating to bending strain; ii) reaction of the tilted grating to temperature; b) processing the response of the tilted grating to distinguish the reaction of the tilted grating to bending strain from the reaction of the tilted grating to temperature. 71) Apparatus for measuring bending strain, comprising: a) a bending strain sensor, having: i) an optical pathway; ii) a tilted grating in said optical pathway to generate a response conveying information on bending strain and temperature acting on said tilted grating; b) a signal processing unit to process the response of the tilted grating and distinguish in the response to reaction of said tilted grating to bending strain from the reaction of the tilted grating to temperature. 72) Apparatus as defined in claim 71 wherein said optical pathway includes an optical fiber. 73) Apparatus as defined in claim in claim 71, wherein the response of said tilted grating includes a core mode resonance component and a cladding mode resonances component. 74) A sensor as defined in claim 1, wherein the electromagnetic radiation propagates in said optical pathway along a propagation direction, said sensor including a reflector in said optical pathway to reflect the response of said tilted grating. 75) A sensor as defined in claim 74, wherein said reflector directs the response to travel in said optical pathway in a direction opposite to the direction of propagation. 76) A sensor as defined in claim 75, wherein said optical pathway includes a straight termination, said reflector being at said termination. 77) A sensor as defined in claim 76, wherein said reflector provides reflectivity through Fresnel reflection. 78) A sensor as defined in claim 76, wherein said reflector includes a reflective coating deposited on said straight termination. 79) A sensor as defined in claim 75, wherein said reflector includes a grating other than said tilted grating. 80) A sensor as defined in claim 79, wherein said grating other than said tilted grating is a non-tilted grating. 81) A sensor as defined in claim 80, wherein the response of said tilted grating includes a plurality of cladding mode resonances, said grating other than said tilted grating has a reflection spectrum that overlaps two or more of said cladding mode resonances. 82) A pressure sensor, comprising: a) an optical pathway; b) a tilted grating in said optical pathway to generate a response conveying information on bending strain acting on said sensor; c) a flexible member, said optical pathway being mounted to said flexible member, said flexible member flexing in response to pressurized fluid and inducing in said tilted grating bending strain. 83) Pressure sensor as defined in claim 82 wherein said optical pathway includes an optical fiber. 84) Pressure sensor as defined in claim in claim 82, wherein the response of said tilted grating includes a core mode resonance component and a cladding mode resonances component. 85) A sensor, for sensing at least one physical manifestation occurring in a medium, said sensor comprising: a) an optical pathway; b) an interface coupled with said optical pathway, said interface being responsive to the physical manifestation to induce strain on said optical pathway; c) a tilted grating in said optical pathway, said tilted grating being responsive electromagnetic radiation propagating in said optical pathway to generate a response conveying information on the strain induced on said optical pathway. 86) A sensor as defined in claim 85, wherein said interface includes a coating on said optical pathway. 87) A sensor as defined in claim 86, wherein said coating induces elongation strain on said optical pathway. 88) A sensor as defined in claim 87, wherein said optical pathway includes an optical fiber. 89) A sensor as defined in claim 88, wherein said coating extends continuously around said optical fiber. 90) A sensor as defined in claim 86, wherein said coating induces bending strain on said optical pathway. 91) A sensor as defined in claim 90, wherein said optical pathway includes an optical fiber. 92) A sensor as defined in claim 91, wherein said coating extends partially around said optical fiber. 93) A sensor as defined in claim 86, wherein said coating swells in response to humidity to in turn induce strain on said optical pathway. 94) The sensor of claim 28 further comprising a self-assembling monolayer printed onto the sensing surface of the sensor, wherein the self-assembling monolayer has a recognition analyte and wherein the recognition analyte associates with a chemical or biological analyte. 95) The sensor of claim 94, wherein the chemical or biological analyte is a microorganism. 96) The sensor of claim 94, wherein the chemical or the biological analyte is a nucleic acid. 97) The sensor of claim 94, wherein the chemical or the biological analyte is a bacteria. 98) The sensor of claim 94, wherein the chemical or the biological analyte is a virus. 99) Use of the sensor of claim 94 for immunoassay methods. 100) A sensor as defined in claim 29, wherein said sensor is a viral sensor. 101) A sensor as defined in claim 29, wherein said sensor is a nucleic acid sensor. 102) A method for detecting the presence of a chemical or biological element in a medium, comprising: a) providing a sensor having a sensing surface, said sensor having an optical pathway containing a tilted grating; b) placing the sensing surface in contact with the medium; determining from a response of said sensor if the chemical or biological element is present in the medium. 103) A method as defined in claim 102, wherein the chemical or biological element is a microorganism. 104) A method as defined in claim 102, wherein the chemical or biological element is a nucleic acid. 105) A sensor as defined in claim 1, wherein the tilted grating has a tilted angle in the range from greater than 0 to less than 45 degrees. 106) A sensor as defined in claim 1, wherein the tilted grating has a tilted angle in the range from greater than 0 to about 20 degrees. 107) A sensor as defined in claim 1, wherein the tilted grating has a tilted angle in the range from about 2 degrees to 12 degrees. 